Information records containing video, audio and color information in the form of a very fine surface relief pattern in a plastic disc are known. The surface relief pattern is monitored by a playback stylus and the surface variations are reconstituted in electrical signal form and converted back to information suitable for display by a television receiver.
In a system described by Clemens in U.S. Pat. No. 3,842,194, incorporated herein by reference, a capacitance system is employed whereby the disc and the playback stylus are electrically conducting and a dielectric layer is between them. Initially a plastic disc was coated with a thin metal layer and then a thin dielectric layer. The need to provide two layers produced a system that was cumbersome and expensive and led to a search for a conductive molding composition from which a conductive molded disc could be made.
Fox et al in copending application, Ser. No. 818,279 filed July 25, 1977, have described conductive video disc replicas made by molding a plastic molding composition containing sufficient finely divided conductive particles so that the material has a bulk resistivity below about 500, and preferably below 100 ohm-cm at 900 megahertz. They disclosed that conductive compression molded video discs could be made using a polyvinylchloride homopolymer of copolymer resin containing stabilizers, lubricants and processing aids and conductive particles having a low bulk density in an amount sufficient to produce the required conductivity.
The original molding compositions made by Fox et al were stiff and difficult to process and thus a search was undertaken to improve the processing of the molding composition. In an application filed by Khanna, Ser. No. 818,716, also filed July 25, 1977, now U.S. Pat. No. 4,151,132 a conductive molding composition is described that has improved processability and produces video discs which are less brittle and display reduced bleed out of additives or staining. The latter is undesirable because it changes the surface characteristics of the video discs, producing a high noise level. Khanna's solution was to mix small amounts of a large number of additives, including lubricants, stabilizers and processing aids, into a mixture of vinylchloride resins. Accordingly, Khanna's composition contained about 12-20 percent by weight of conductive carbon black, about 10 percent by weight of a vinyl chloride-vinyl acetate copolymer, about 10 percent by weight of a vinyl chloride-maleate ester copolymer, about 15-17 percent by weight of four or more processing aids and plasticizers, about 3.5 percent by weight of at least two metal stabilizers, about 1.5 percent by weight of at least two lubricants, the balance being a vinyl chloride-propylene copolymer.
The molding compositions of Khanna are readily processable and moldable to form video discs of excellent playback characteristics but they are unsatisfactory from the standpoint of high temperature storage. These compositions have a comparatively low heat distortion temperature which results in permanent deformation or warpage and shrinkage of the discs when stored at temperatures above about 100.degree. F. (37.8.degree. C.).
According to present requirements, a conductive molding composition for the video disc application must be able to provide a flat, rigid disc with excellent replication of minute video information which is dimensionally stable on storage at temperature up to 130.degree. F. (54.4.degree. C.). The present standard for the video disc requires that it have a vertical distortion or warp over the entire surface of a 12 inch (30.5 cm) disc of 0.02 inch (0.05 cm) or less when stored at 130.degree. F. for up to 48 hours, and a lateral shrinkage of no more than 0.05 inch (0.13 cm). Khanna's compositions have been unable to meet these requirements.
Thus a search has continued to obtain conductive molding compositions which have good processability and thermal stability during compound processing and molding; good replication of submicron size surface relief patterns; and dimensional stability on storage under various environmental conditions up to 130.degree. F. (54.4.degree. C.).